Induction Motors Fed by PWM (WEG)

7/27/2019 Induction Motors Fed by PWM (WEG)

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Motors | Energy | Automation | Coatings

Induction motors fed by

PWM frequency inverters

Technical guide


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Technical guide Induction motors fed by PWM frequency inverters2

Table of contents

1 Introduction........................................................................................................................................................................................... 4

2 Normative Aspects.............................................................................................................................................................................. 52.1 NEMA MG1 - Motors and generators / United States............................................................................................................... 5

2.2 NEMA - Application Guide for AC Adjustable Speed Drive Systems........................................................................................ 5

2.3 IEC 60034 - Rotating Electrical Machines / Internacional........................................................................................................ 5

2.4 Other technical documents of reference......................................................................................................................................... 5

3 Induction machines speed variation................................................................................................................................................. 5

4 Characteristics of PWM frequency inverters............................................................................................................................... 7

4.1 General................................................................................................................................................................................................... 7

4.2 Control Types....................................................................................................................................................................................... 8

5 Interaction between inverter and AC power line............................................................................................................................ 8

5.1 Harmonics............................................................................................................................................................................................. 85.1.1 Normative considerations about the harmonics............................................................................................................................ 9

5.2 Line reactor / DC bus choke.............................................................................................................................................................. 9

6 Interaction between inverter and motor...................................................................................................................................... 10

6.1 Harmonics influencing motor performance...................................................................................................................................10

6.1.1 Normative considerations about the inverter output harmonics.......................................................................................... 10

6.2 Considerations regarding energy efficiency.................................................................................................................................. 11

6.2.1 The influence of the speed variation on the motor efficiency.................................................................................................... 12

6.2.2 Normative considerations about the efficiency of inverter fed motors................................................................................ 12

6.3 Influence of the inverter on the temperature rise of the windings......................................................................................... 13

6.4 Criteria regarding the temperature rise of WEG motors on VSD applications....................................................................... 136.4.1 Torque derating...................................................................................................................................................................................13

6.4.2 Breakaway torque.............................................................................................................................................................................. 14

6.4.3 Breakdown torque............................................................................................................................................................................. 15

6.5 Influence of the inverter on the insulation system.................................................................................................................... 15

6.5.1 Rise Time............................................................................................................................................................................................ 15

6.5.2 Cable length........................................................................................................................................................................................ 16

6.5.3 Minimum time between successive pulses (MTBP).................................................................................................................... 17

6.5.4 Switching frequency (fs).................................................................................................................................................................... 18

6.5.5 Multiple motors................................................................................................................................................................................... 18

6.6 Criteria regarding the insulation system of WEG motors on VSD applications......................................................................186.7 Normative considerations about the insulation system of inverter fed motors.................................................................. 18

6.8 Recommendations for the cables connecting WEG motors to inverters............................................................................ 19

6.8.1 Cable types and instal lation recommendations.......................................................................................................................... 20

6.9 Influence of the inverter on the motor shaft voltage and bear ing currents......................................................................... 20

6.9.1 Common mode voltage.....................................................................................................................................................................21

6.9.2 Equivalent circuit of the motor for the high frequency capacitive currents............................................................................ 21

6.9.3 Methods to reduce (or mitigate) the bearings currents in inverter fed motors................................................................... 22

6.10 Criteria regarding protection against bearing currents (shaft voltage) of WEG motors on VSD applications................. 23

6.11 Normative considerations about the current flowing through the bearings of inverter fed motors............................... 23

6.12 Influence of the inverter on the motor acoustic noise............................................................................................................. 23

6.13 Criteria regarding the noise emitted by WEG motors on VSD applications.......................................................................... 23

6.14 Normative considerations about the noise of inverter fed motors....................................................................................... 24


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The number of industry applications in which inductionmotors are fed by static frequency inverters is growing fastand, although much has already been done within this field,there is still a lot to be studied/understood regarding suchapplications. The advance of variable speed drives systemsengineering increasingly leads to the need of specifictechnical guidance provision by electrical machines anddrives manufacturers, so that such applications can besuitably designed in order to present actual advantages interms of both energy efficiency and costs.

This technical guide aims to clarif y the main aspectsconcerning applications of low voltage ( 690 V) induction

motors with static frequency inverters supply, for frames IEC 355 (NEMA 587), in a didactic and concise approach.

First of all the principal and most broadly followedinternational standards about the subject are mentioned.

Then the theoretical basis of speed variation on inductionmachines by means of indirect static inverters is presented,as well as the fundamental characteristics of electronicinverters.

Once the basics of adjustable speed drives are known, thebehavior of the whole power system is analyzed. Each

component of the power system (AC power line - frequencyinverter - induction motor - load) is focused, as well as theoverall interactions between them, resulting from speedvariation. In this manner the whole drive system can be wellunderstood.

At last examples of VSD systems designs are presented, for abetter understanding of the matters exposed throughout thedocument.

Always looking out for a technical elucidation as complete aspossible along this guide, some controversial points areemphasized. Divergences existing among distinct

standardization organisms are discussed and WEGs point ofview is explained.

1 Introduction


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Technical guide Induction motors fed by PWM frequency inverters 5

For an induction motor, rotor speed, frequency of the voltagesource, number of poles and slip are interrelated accordingto the following equation:

where:n : mechanical speed (rpm)1

: fundamental frequency of the input voltage (Hz)p : number of poless : slip

The analysis of the formula above shows that the mechanicalspeed of an induction motor is a function of threeparameters. Thus the change of any of those parameters willcause the motor speed to vary as per the table below.

2 Normative Aspects

3 Induction machines speed variation

2.1 NEMA MG1 - Motors and generators / United

States

Parte 30 - Application considerations for constant speedmotors used on a sinusoidal bus with harmonic contentand general purpose motors used with adjustable-frequency controls or both (2006)

Parte 31 - Definite-purpose inverter-fed polyphase motor(2006)

2.2 NEMA - Application Guide for AC Adjustable

Speed Drive Systems (2001)

2.3 IEC 60034 - Rotating Electrical Machines /

International

Parte 17- Cage induction motors when fed from inverters application guide (2006)

Parte 25 - Guide for the design and performance of cageinduction motors specifically designed for inverter supply(2007)

2.4 Other technical documents of reference

GAMBICA/REMA Technical Guides for Variable SpeedDrives and Motors

GAMBICA/REMA Technical Reports for Variable SpeedDrives and Motors

CSA C22.2 No.100-2004 Item 12 (Canada) Motors andGenerators Industrial Products

JEM-TR 148-1986 (Japan) Application guide for inverterdrive (general-purpose inverter)

IEC 60034-18-41 Qualification and design tests for Type Ielectrical insulation systems used in rotating electricalmachines fed from voltage inverters

Papers and books related to this subject

Speed variation

Parameter Application characteristics

Number of polesDiscrete variation

Oversizing

Slip

Continuous variation

Rotor losses

Limited frequency range

Voltage frequencyContinuous variation

Utilization of STATIC FREQUENCY Inverters!

n = 120 f1(1-s)

p


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The utilization of static frequency inver ters comprehendscurrently the most efficient method to control the speed ofinduction motors. Inverters transform a constant frequency-constant amplitude voltage into a variable (controllable)frequency-variable (controllable) amplitude voltage. Thevariation of the power frequency supplied to the motor leadsto the variation of the rotating field speed, which modifies themechanical speed of the machine.

The torque developed by the induction motor follows theequation below:

Despising the voltage drop caused by the stator impedance,the magnetizing flux is found to be:

where:T: torque available on the shaft (N.m)m : magnetizing flux (Wb)I

2: rotor current (A) depends on the load!

V1

: stator voltage (V)k

1e k

2: constants depend on the material and on the

machine design!

Considering a constant torque load and admitting that thecurrent depends on load (therefore practically constantcurrent), then varying proportionally amplitude and frequencyof the voltage supplied to the motor results in constant flux

and therefore constant torque while the current remainsunchanged. So the motor provides continuous adjustmentsof speed and torque with regard to the mechanical load.Losses can be thus minimized in accordance with the loadconditions by keeping the slip constant at any speed, for agiven load.

The curves below are obtained from the equations above.

The ratio V1/f1 is kept constant up to the motor base (rated)frequency. From this frequency upwards the voltage is keptconstant at its base (rated) value, while the frequency appliedon the stator windings keeps growing, as shown next.

Thereby the region above the base frequency is referred to asfield weakening, in which the flux decreases as a result offrequency increase, causing the motor torque to decreasegradually. The typical torque versus speed curve of an

inverter fed induction motor is illustrated below.

The number of variable speed applications controlled bymeans of a frequency inverter has increased significantly overthe recent years. This may be explained by the many benefitsprovided by such applications: Aloof control the control can be installed remotely at a

suitable location, keeping just the motor in the processingarea on the contrary of hydraulic and mechanical varyingspeed systems.

Aloof control the control can be installed remotely at asuitable location, keeping just the motor in the processingarea on the contrary of hydraulic and mechanical varyingspeed systems.

Cost reduction direct on line startings of induction motorscause current peaks that harm the motor as well as otherelectric equipments linked to the electrical system. Staticfrequency inverters provide softer startings, resulting incost reduction with regard to maintenance.

Gain of productivity industrial systems are often oversizeddue to an expectation of future production increase. Staticinverters allow the proper regulation of the operationalspeed according to the equipments available and theproduction needs.

Energy Efficiency the power system global efficiency

depends not only on the motor, but also on the control.Static inverters are high efficiency apparatuses, reachingtypically 97% or more. Induction motors also present highefficiency levels, reaching up to 95% or even more in larger

m= k2 .V1f1

T= k1 .m . I2

It comes out that torque is kept constant up to the basefrequency and beyond this point it falls down (weakeningfield). Since the output is proportional to torque timesspeed, it grows linearly up to the base frequency and fromthat point upwards it is kept constant. This is summarizedby the graph beside.


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machines operating at rated conditions. When speedvariation is required, the output changes in an optimizedway, directly affecting the energy consumption and leadingto high efficiency levels performed by the system (inverter +motor).

Versatility static frequency inverters suit both variable andconstant torque loads. With variable torque loads (lowtorque demand at low speeds) the motor voltage isdecreased to compensate for the efficiency reductionnormally resultant from load reduction. With constanttorque (or constant power) loads the system efficiencyimprovement comes from the feasibility of continuousadjustment of speed, with no need to use multiple motorsor mechanical variable speed systems (such as pulleys andgears), which introduce additional losses.

High quality the accurate speed control obtained withinverters results in process optimization, providing a finalproduct of better quality.Aloof control the control can be installed remotely at asuitable location, keeping just the motor in the processingarea on the contrary of hydraulic and mechanical varyingspeed systems.

4 Characteristics of PWM frequency inverters

4.1 GeneralPWM voltage source static frequency inverters presentlycomprehend the most used equipments to feed low voltageindustrial motors in applications that involve speed variation.They work as an interface between the energy source (ACpower line) and the induction motor.

In order to obtain an output signal of desired voltage andfrequency, the input signal must accomplish three stageswithin a frequency inverter:

Diode bridge - Rectification of the AC input voltage -

constant amplitude and frequency - coming from thepower grid;

DC link or filter- Regulation/smoothing of the rectifiedsignal with energy storage through a capacitor bank;

IGBT power transistors Inversion of the voltage comingfrom the link DC into an alternate signal of variableamplitude and frequency.

The following diagram depicts the three stages of an indirectfrequency inverter.

Rectifier

Input:50/60 Hz (1 or 3 ) Output:Variable voltageand frequency

AC AC

DC

Filter Inverter| motor

VPWM

Motor3

VDC 1.35 Vin or 1.41 Vin

Vin


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NOTES: Under light load (or at no load) conditions, the DC link

voltage tends to stabilize at

However, when the motor drives heavier loads (for

instance, at full load), the DC link voltage tends to the value

The criteria used to define the insulation system of WEGmotors fed by inverters, presented further on, consider thehighest of those values (1.41Vin), which is more critical tothe motor. In this way WEG motors attend both situationssatisfactorily.

2 Vrede 1,41 Vrede

2 Vrede 1,35 Vrede(3/ )

4.2 Control Types

There are basically two inverter control types: scalar (openloop) and vector (open or closed loop).

The scalar controlis based on the original concept of afrequency inverter: a signal of certain voltage/frequency ratiois imposed onto the motor terminals and this ratio is keptconstant throughout a frequency range, in order to keep themagnetizing flux of the motor practically unchanged. It isgenerally applied when there is no need of fast responses totorque and speed commands and is particularly interestingwhen there are multiple motors connected to a single drive.The control is open loop and the speed precision obtained isa function of the motor slip, which depends on the load,since the frequency is imposed on the stator windings. Inorder to improve the performance of the motor at low

speeds, some drives make use of special functions such asslip compensation (attenuation of the speed variation asfunction of load) and torque boost (increase of the V/f ratio to

5.1 Harmonics

For the AC power line, the system (frequency inverter +motor) is a non-linear load whose current include harmonics(frequency components multiples of the power line

frequency). The characteristic harmonics generally producedby the rectifier are considered to be of order h = np1 on theAC side, that is, on the power line (p is the number of pulsesof the inverter and n =1,2,3). Thus, in the case of a 6 diode (6pulses) bridge, the most pronounced generated harmonicsare the 5th and the 7th ones, whose magnitudes may varyfrom 10% to 40% of the fundamental component, dependingon the power line impedance. In the case of rectifyingbridges of 12 pulses (12 diodes), the most harmful harmonicsgenerated are the 11th and the 13th ones. The higher theorder of the harmonic, the lower can be considered itsmagnitude, so higher order harmonics can be filtered moreeasily. As the majority of drives manufacturers, WEG

produces its low voltage standard inverters with 6-pulserectifiers.

compensate for the voltage drop due to the statorresistance), so that the torque capacity of the motor ismaintained. This is the most used control type owing to itssimplicity and also to the fact that the majority of applicationsdo not require high precision or fast responses of the speedcontrol.

The vector controlenables fast responses and high level ofprecision on the motor speed and torque control. Essentiallythe motor current is decoupled into two vectors, one toproduce the magnetizing flux and the other to producetorque, each of them regulated separately. It can be openloop (sensorless) or closed loop (feedback).

Speed feedback a speed sensor (for instance, anincremental encoder) is required on the motor. This controlmode provides great accuracy on both torque and speed

of the motor even at very low (and zero) speeds. Sensorless simpler than the closed loop control, but its

action is limited particularly at very low speeds. At higherspeeds this control mode is practically as good as thefeedback vector control.

The main difference between the two control types is that thescalar control considers only the magnitudes of theinstantaneous electrical quantities (magnetic flux, current andvoltage) referred to the stator, with equations based on theequivalent electrical circuit of the motor, that is, steady stateequations. On the other hand, the vector control considersthe instantaneous electrical quantities referred to the rotor

linkage flux as vectors and its equations are based on thespatial dynamic model of the motor. The induction motor isseen by the vector control as a DC motor, with torque andflux separately controlled.

5 Interaction between inverter and AC power line

The power system harmonic distortion can be quantified bythe THD (Total Harmonic Distortion), which is informed by theinverter manufacturer and is defined as:

where:Ah are the rms values of the non-fundamental harmoniccomponentsA1 is the rms value of the fundamental component

The waveform above is the input measured current of a6-pulse PWM inverter connected to a low impedance powergrid.

THD =h = 2

8

2An

A1


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3.2..fline .Irated

5.1.1 Normative considerations about the harmonics

The NEMA Application Guide for AC ASD Systems refers toIEEE Std.519 (1992), which recommends maximum THDlevels for power systems 69 kV as per the tables presentednext. This standard defines final installation values, so thateach case deserves a particular evaluation. Data like thepower line short-circuit impedance, points of commonconnection (PCC) of inverter and other loads, among others,influence on the recommended values.

5.2 Line reactor / DC bus choke

Harmonic currents, which circulate through the power lineimpedances and depend on the rectifier input/outputimpedance values, cause harmonic voltage drops that distortthe power supply voltage of the inverter and other loads

connected to this line. These harmonic current and voltagedistortions may increase the electrical losses in theinstallation, lowering the power factor and overheatingcomponents such as cables, transformers, capacitor banks,motors, etc.

The addition of a line reactor and/or a DC bus choke reducesthe harmonic content of the current and increase the powerfactor. The DC bus choke has the advantage of notintroducing a motor voltage drop but depending on thecombination of its value with the power line impedance andthe DC link capacitance values it may result in undesirableresonances within the overall system. On the other hand, the

line reactor decreases the medium voltage of theintermediate circuit but attenuates more effectively powersupply voltage transients. Besides that, it extends thesemiconductors and the DC link capacitor bank lifetimes, as

The maximum harmonic current distortion recommended byIEEE-519 is given in terms of TDD (Total Demand Distortion)and depends on the ratio (ISC / IL), where:ISC = maximum short-current current at PCC.IL = maximum demand load current (fundamental frequencycomponent) at PCC.

The documents mentioned from IEC, however, do not setlimits for the harmonic distortion injected by inverters into thepower line.

L =(voltage drop)

%. V

lineH

Voltage harmonics

Even components 3,0%

Odd components 3,0%

THDvoltage

5,0%

Individual Odd Harmonics

(Even harmonics are limited to 25% of the odd harmonic limits)

Maximum harmonic current distortion in percent of IL

ISC / IL < 11 11 h

17

17 h

23

23 h

35

35 h TDD

< 20* 4.0 2.0 1.5 0.6 0.3 5.0

20 < 50 7.0 3.5 2.5 1.0 0.5 8.0

50 < 100 10.0 4.5 4.0 1.5 0.7 12.0

100 < 1000 12.0 5.5 5.0 2.0 1.0 15.0

> 1000 15.0 7.0 6.0 2.5 1.4 20.0

* All power generation equipment is limited to these values of current distortion,

regardless of actua l ISC / IL.

(a)

(a)

(b)

(b)

a result of the decrease of both the rms current of therectifying diodes and the current ripple through the middlecircuit capacitors.

The value of the line reactor needed for the desired voltagedrop to be obtained can be calculated as follows:

[ ]

Current and voltage waveforms with (b) and without (a) linereactor. It can be seen that line reactors soften the peaks,thus reducing the harmonic content and the rms value of theinput current. Additionally, diminution of the supply voltagewaveform distortion is thereby caused.A minimum line impedance that introduces a voltage dropfrom 1 to 2%, depending on the inverter size, isrecommended in order to ensure the inverter lifetime.

As rule of thumb, it is recommended to add a line reactor tothe existing power supply impedance (including transformersand cables) so that a total voltage drop of 2 to 4% isachieved. This practice is considered to result in a goodcompromise between motor voltage drop, power factorimprovement and harmonic current distortion reduction.

The (a) line reactor and (b) DC bus choke electricalinstallations are shown next.

(a) Input line reactor connection

Converter input current

Converter input voltage


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Then the motor fed by frequency inverter sees a pulsating(PWM) voltage and a practically sinusoidal current, so that

the voltage harmonics generally present higher magnitudesthan the current harmonics.

* All frequency inverters manufactured by WEG employ Space Vector Modulation.

6.1.1 Normative considerations about the inverter

output harmonics

There is no international standardization defining maximumacceptable values for voltage and current harmonicdistortion. However, the international standards do considerthe increase of motor losses due to the non-sinusoidalsupply.

IEC 60034-17 provides an example of motor losses increaseowing to PWM supply. Motor info: 315 IEC frame, ratedtorque and speed values.

6.1 Harmonics influencing motor performance

The induction motor, when under PWM voltage coming fromthe inverter, is subjected to voltage harmonics (frequencycomponents above the fundamental frequency). Dependingon the type of PWM employed, the switching frequency andother peculiarities of the control, the motor may presentefficiency decrease and losses, temperature, noise and

vibration levels increase.

Furthermore other effects may appear when inductionmotors are fed by inverters. Insulation system dielectric stressand shaft voltages allied with potentially damaging bearingcurrents are well known side effects. Although not producedspecifically by harmonics but by other matters that will soonbe approached, these are important effects and should notbe neglected. The motor current and voltage waveformswhen under PWM supply are illustrated below.

6 Interaction between inverter andmotor

PWM voltage at the

inverter outputInverter fed motor current

There are basically the following solutions to mitigate theharmonics generated by a PWM frequency inverter:

Methods o f reduc tion o f harmonics Solu tion charac te ris ti cs

Installation of output passive filters(L, LC (sinusoidal), dV/dt)

Installation costs increase

Restrictions for vector control operation

Voltage drop (motor horsepower

reduction)

Use of multi-level inverters

Costs increase

Inverter reliability decrease

Control complexity increase

Pulse Width Modulation quality

improvement (optimization of pulse

patterns)

Space Vector Modulation (SVM)*

Do not increase costs

Voltage control upgrade

Higher system (inverter + motor)

efficiency

Switching frequency increase

Inverter efficiency decrease (higher

switching losses)

- Common mode leakage current flow

increase

(b) DC bus choke connection


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6.2 Considerations regarding energy efficiency

The lack of international standards that specify test

procedures to evaluate the system (motor + inverter)efficiency allows such tests to be carried out in many differentand non contestable ways. Therefore, the results obtainedshould not influence the acceptance (or not) of the motor,except under mutual accordance between customer andmanufacturer. Experience shows the effectiveness of theconsiderations below. An induction motor fed by PWM voltage presents a lower

efficiency level than when fed by purely sinusoidal voltage,due to the losses increase caused by harmonics;

Anyway, when induction motors are fed by static inverters,the efficiency of the overall system, rather than the motorefficiency only, should be evaluated;

Each case must be properly analyzed, taking into accountcharacteristics of both the motor and the inverter, such as:operating frequency, switching frequency, speed range,load conditions and motor power, THD, etc.

The measuring instrumentation is extremely important forthe correct evaluation of electrical quantities on systemsunder PWM duty. True RMS meters must be used, in orderto permit reliable measurements of power;

Higher switching frequencies increase the motor efficiencyand decrease the inverter efficiency (due to the increase ofcommutation losses).

High efficiency motors keep their efficiency higher,

compared to standard motors, when both are fed byinverters.

Losses caused by fundamental frequency

Losses caused by harmonics

A Stator winding lossesB Rotor winding lossesC Iron lossesD Additional load lossesE Frictional losses

F Stator winding lossesG Rotor winding lossesH Iron losses

I Additional load lossesJ Commutation losses

NOTE: frame 315 (IEC) motor operating at rated speed andtorque.

IEC 60034-25 illustrates the motor losses increase due toPWM supply by means of the following curves:

NEMA MG1 Part 30 considers a derating factor (torquereduction) to avoid excessive overheating of a general

purpose motor fed by converter, compensating for thecirculation of harmonic currents due to the PWM voltageharmonic content:

Where:n: order of the odd harmonic, not including those divisible bythreeVn: per unit magnitude of the voltage at the nth harmonicfrequency

HVF =n = 5

8

2Vn

n


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6.2.1 The influence of the speed variation on the motor

efficiency

The effects of speed var iation on the motor efficiency can beunderstood from the analysis of the behavior of the inverterfed motor output power as a function of its operation speed.

6.2.2.1 Numerical example

6.2.2 Normative considerations about the efficiency of

inverter fed motors

Supposing, for instance, a 60 Hz frequency base for thesituations outlined above:

Some practical values found by means of the input-outputmeasurement method are shown below for standard motors:

NEMA MG1 Part 30 Efficiency will be reduced when amotor is operated on a bus with harmonic content. The

harmonics present will increase the electrical losses which,in turn, decrease efficiency. This increase in losses will alsoresult in an increase in motor temperature, which furtherreduces efficiency.

Motor 75 HP (55 kW) 6 poles 400 V 50 Hz

conv= P2 /P1

conv= P3 /P2sist= Pout/Pin =P3/P1 = conv.mot

And, according to the exposed above,

Then the following situation results from speed reduction:

Considering that the motor losses are essentially comprisedof Joule losses (PJ) and iron losses (PI) and assuming that theJoule losses prevail, then the motor efficiency fall at lowspeeds, where the motor output power is reduced and,despite the slight decrease of the iron losses (frequencydependant), the Joule losses (current square dependant) arekept nearly constant for a constant torque load, so that afterall there is no significant variation of the overall losses.

The equations next explain that. Defining efficiency as:

P60Hz= Pu

P30Hz

= Pu= 0,5 P

u

60

30

% =

%

=

Pout

Pout

Pout

Pout

+

Losses

Losses P

J+

P

iron

Losses constantP

iron

+

PJ

constant (PJ

>> Piron

)

(PJ

>P

iron)

Pin

}

}

Motor 15 HP (11 kW) 4 poles 400 V 50 Hz


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6.4 Criteria regarding the temperature rise of WEG

motors on VSD applications

6.4.1 Torque deratingIn order to keep the temperature rise of WEG motors, whenunder PWM supply, within acceptable levels, the followingloadability limits must be attended (observe themotor lineand the flux condition).NOTE: Applications with motors rated for use in hazardousareas must be particularly evaluated - in such case pleasecontact WEG.

6.4.1.1 Para motores do mercado NEMA

6.3 Influence of the inverter on the temperature rise of

the windings

Induction motors may heat up more when fed by frequencyinverter than when fed by sinusoidal supply. This higher

temperature rise results from the motor losses growth owingto the high frequency components of the PWM signal and theoften reduced heat transfer resulting from speed variation.

The voltage harmonic distortion contributes to increase themotor losses, once that creates minor hysteretic loops in thelamination steel, increasing the effective saturation of themagnetic core and giving rise to high frequency harmoniccurrents, which bring about additional Joule losses.Nevertheless, these high frequency components do notcontribute to the production of torque at steady operation ofthe motor, since they do not increase the airgap fundamentalflux, which rotates at synchronous speed. The operation at

low speeds causes the ventilation over the (self-ventilated)motor frame to decrease, consequently lowering the motorcooling and raising in this way the thermal stabilizationtemperature.

NEMA MG1 Part 31 Performance tests, when required,shall be conducted on a sinusoidal power supply unlessotherwise specified by mutual agreement between themanufacturer and the user.

NEMA Application Guide for AC ASD Systems The overall

efficiency of an ASD is based on the total losses of thecontrol, the motor, and any auxi liary equipment. (...) Themotor efficiency when operated on a control is slightly lessthan when operated on sinewave power. Overall systemefficiency is often increased when used an ASD. Traditionalmethods of changing speed such as gears or beltsintroduce additional losses which reduce efficiency.

IEC 60034-17 The performance characteristics andoperating data for drives with inverter-fed cage inductionmotors are influenced by the complete system, comprisingsupply system, inverter, induction motor, mechanicalshafting and control equipment. Each of these componentsexists in numerous technical types. Any values quoted in

this technical specification are thus indicative only. (...)There is no simple method to calculate the additionallosses and no general statement can be made about theirvalue. Their dependence upon the different physicalquantities is very complex. Also there is a great variety bothof inverters and of motors.

IEC 60034-25 The recommended methods to determinethe motor efficiency are given in IEC 60034-2 (summation-of-losses method for motors > 150 kW and input-outputmeasurement for motors 150 kW). The no-load losses(including the additional losses) should be measured at thesame pulse pattern and pulse frequency that the inverterwill produce at rated load. The determination of the overall

efficiency of the system (motor + inverter) by means ofinput-output measurement for motors > 150 kW is alsoapplicable under agreement between manufacturer anduser. In this case, however, the motor efficiency shall not bedetermined separately.

Therefore, when operating with frequency inverters, both theeffects mentioned above must be considered. There arebasically the following solutions to avoid excessiveoverheating of the inverter fed motor: Torque derating (oversizing of the self ventilated motor

frame); Utilization of independent cooling system (separate

ventilation); Utilization of the Optimal Flux Solution (exclusive to

applications using WEG drives and motors).

TEFC W21 and W22 (High Efficiency) motors

Frame SizeConstant

Torque

Variable

Torque

Constant

PowerDrive Comments

143

587(***)

12:1 1000:1 60 120 Hz AnyConstant

flux

100:1(*) - 60 120 Hz WEG(**) Optimal flux

587(****)4:1 1000:1 60 120 Hz Any

Constant

flux

10:1 - 60 120 Hz WEG(**) Optimal flux

TEFC NEMA PREMIUM EFFICIENCY motors

Frame Size

Constant

Torque

Variable

Torque

Constant

Power Drive Comments

143

587(***)

20:1 1000:1 60 120 Hz AnyConstant

flux

1000:1(*) - 60 120 Hz WEG(**) Optimal flux

587(****)6:1 1000:1 60 120 Hz Any

Constant

flux

12:1 - 60 120 Hz WEG(**) Optimal flux

(*)Satisfactory motor performance depends on proper dri ve setup please contact WEG

(**)WEG drive CFW-09 version 2.40 or higher, operating in sensorless (open loop) vector

mode

(***)Motors with rated power 250 hp. Criteria also valid for motors of the frame sizes 447and 449

(****)Motors with rated power > 250 hp. Criteria also valid for motors of the frame sizes 447

and 449


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6.4.3 Breakdown torque

Above base speed the motor voltage must be kept constantfor constant power operation, as already shown. NEMA MG1Part 31 prescribes that the breakdown torque at any frequen-cy within the defined frequency range shall be not less than

150% of the rated torque at that frequency when rated volt-age for that frequency is applied. WEG motors when fed byinverters satisfy such criterion up to 90 Hz.

The maximum torque capability of the motor (breakdowntorque) limits the maximum operating speed in which con-stant power operation is possible. Attending NEMA recom-mendations, one can approximately find this limit from the fol-lowing equation:

6.5 Influence of the inverter on the insulation system

The evolution of the power semiconductors have led to thecreation of more efficient, but also faster, electronic switches.The high switching frequencies of the IGBT transistorsemployed in modern frequency inverters bring about someundesirable effects, such as the increase of electromagneticemission and the possibility of voltage peaks, as well as highdV/dt ratios (time derivative of the voltage, that is, rate ofelectrical potential rise), occurrence at the inverter fed motorterminals. Depending on the control characteristics (gate

resistors, capacitors, command voltages, etc.) and the PWMadopted, when squirrel cage induction motors are fed byfrequency inverters, those pulses combined with theimpedances of both the cable and the motor may causerepetitive overvoltages on the motor terminals. This pulsetrain may degrade the motor insulation system and mayhence reduce the motor lifetime.

The cable and the motor can be considered a resonantcircuit, which is excited by the inverter rectangular pulses.When the values of R, L and C are such that the peak voltageexceeds the supply voltage (VDC 1.41 Vin), the circuitresponse to this excitation is a so called overshoot. The

overshoots affect especially the interturn insulation of randomwindings and depend on several factors: rise time of thevoltage pulse, cable length and type, minimum time

between successive pulses, switching frequency and

multimotor operation.

6.5.1 Rise Time

The PWM voltage takes some time to rise from its minimumto its maximum value. This period is often called rise time.Due to the great rapidity of switching on the inverter stage,the growth of the voltage wavefront takes place too fast and,

with the power electronics advance, these transition timestend to be more and more reduced.

6.5.1.1 Normative considerations about rise time

The definitions of rise time (tr) according to NEMA and to IECStandards differ, as shown below, allowing for interpretationdivergences and conflicts between manufacturers and usersof motors and drives.

tr: time needed for the voltage to rise from 10 to 90% of theDC link voltage (1.41Vrated)

RPMmax = 2 Tmax RPMbase3

Then the inverter fed motor is subjected to extremely high dV/dt rates, so that the first turn of the first coil of a single phaseis submitted to a high voltage level. Therefore variable speeddrives can considerably increase the voltage stress within amotor coil, though owing to the inductive and capacitive

characteristics of the windings, the pulses are damped onthe subsequent coils.

So the rise time (tr) has a direct influence on the insulation life,because the faster the pulse wavefront grows, the greater thedV/dt ratio over the first coil and the higher the levels ofvoltage between turns, causing the insulation system to wearmore quickly away. Thus the motor insulation system shouldpresent superior dielectric characteristics in order to standthe elevated voltage gradients occurring on PWMenvironment.

NEMA MG1 Part 30

Tbase


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Supposing the motor voltage Vrated = 460 VVlinkDC 1,41 x 460 = 648,6 VV = 0,8 x 648,6 = 518,9 V

Assuming that rise time = 0,1st = 0,1s

Supposing the motor voltage Vrated = 460 Vwith incidence of 1200 V peaks

V = 0,8 x 1200 = 960 V

Assuming tr = 0,25s:

NEMA definition of dV/dt

IEC 60034-25

IEC definition of dV/dt

dV

dV

V

V

V

V

518,9

960

5189

3840

dt

dt

t

t

0,1

0,25

=

=

=

=

=

=

s

s

[

[

[

[

tr: time needed for the voltage to rise from 10% to 90% of thepeak voltage at motor terminals

The signal arriving at the motor through the cable is partiallyreflected, causing overvoltage, because the motor highfrequency impedance is greater than the cable impedance.Excessively long leads increase the overshoots at the motorterminals. According to the NEMA Application Guide for ACASD Systems, with the modern IGBT controls overshootsbegin to occur with a cable length of a few feet and canreach 2 times the control DC bus voltage at a length lessthan 50 feet. In some cases, however, very long cables (inexcess of 400 feet, for example) can result in a situation

where the overshoot does not decay quickly enough. In thiscase the voltage peak at the motor terminals can ring up wellbeyond 2 times the inverter DC link voltage. This behavior is afunction of the PWM pulse pattern, the rise time and the very

NOTE: Due to the cable, the rise time is higher at the motorterminals than at the inverter terminals. However, a verycommon mistake in the dV/dt calculation is to consider therise time at the inverter terminals and the voltage peak at themotor terminals, resulting in an unlikely dV/dt value. Forinstance, considering tr = 0.1s (typical value found at theinverter) in the case above it would result dV/dt = 9600 V/s!

Owing to the differences existing between the rise timedefinitions given by NEMA and IEC, misunderstandings oftenhappen when calculating the voltage gradient (dV/dt).

According to NEMA criterion the DC link voltage ( 1.41 Vin)must be taken as 100% voltage reference for thedetermination of rise time and the calculation of dV/dt.According to IEC criterion, however, the peak voltage arrivingat the motor terminals is what must be taken as 100%voltage reference. Due to the cable, the rise time to beconsidered in IEC criterion will be normally higher than theone considered in NEMA criterion (which is the valueinformed by the inverter manufacturer). Thus depending onthe criteria considered throughout the calculations, prettydifferent values of dV/dt are likely to be attributed to the samesituation.

The insulation criteria defined for WEG motors are based onNEMA, in order not to depend on the final customerinstallation. Furthermore the NEMA criterion seemsappropriate for considering just the linear stretch of the curveto approximate the derivative (dV/dt V/t). The IECcriterion considers the peak voltage at the motor terminals,something extremely complicated to be predicted orestimated a priori. The rise time at the motor terminals isincreased by the cable high frequency impedance. The dV/dtratio at the motor terminals (milder than at the drive terminals)can be also calculated, but it requires a reliable measurementof the voltage pulses at the motor leads and most of timesthis is not easily accomplished or not even feasible,demanding a technician familiar with such applicationsequipped with a good oscilloscope.

6.5.2 Cable length

Beside the rise time, the cable length is a predominant factorinfluencing the voltage peaks occurrence at the inverter fedmotor terminals. The cable can be considered a transmissionline with impedances distributed in sections of inductances/capacitances series/parallel connected. At each pulse, theinverter delivers energy to the cable, charging those reactiveelements.


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Converter terminals 65.5 ft cable

98.5 ft cable 328 ft cable

6.5.2.1 Corona effect

Depending on the quality/homogeneity of the impregnationthe impregnating material may contain voids (cavities), inwhich the failure mechanism of the interturn insulationdevelops. The deterioration of the motor insulating systemdue to the voltage overshoots occurs by means of PartialDischarges (PD), a complex phenomenon resulting fromCorona.

Between adjacent charged conductors there is relativevoltage, which gives rise to an electric field. If the establishedelectric field is high enough (but below the breakdownvoltage of the insulating material), the dielectric strength ofthe air is disrupted, that is, if there is sufficient energy, oxygen(O2) is ionized in ozone (O3). The ozone is highly aggressiveand attacks the organic components of the insulation systemdamaging it. For this to happen though the voltage on theconductors must exceed a threshold value, the so calledCorona Inception Voltage, that is the local breakdownstrength in air (within the void). The CIV depends on thewindings design, insulation type, temperature, superficialcharacteristics and moisture.

6.5.3 Minimum time between successive pulses

(MTBP)

The voltage measurements presented above show that thereis a succession of peaks in the voltage waveform delivered bythe drive and arriving at the motor terminals. This signalpropagates trough the cable at a determined velocity.Depending on the winding characteristics and, with respectto the waveform, on the minimum time between successivepulses, the voltage appearing between turns may vary

sensibly.

The average voltage applied at the motor terminals iscontrolled by the width of the pulses and by the timebetween them. The overshoots get worse with shorter timesbetween pulses. This condition is most likely to occur at highpeak or high output voltages and during transient conditions,such as acceleration or deceleration. If the time betweenpulses is less than three times the resonant period of thecable (typically 0.2 to 2s for industrial cable), then additionalovershoot will occur. The only way to be sure that thiscondition does not exist is by measuring the pulses directlyor by contacting the control manufacturer.

cable type. Voltage measurements realized at the inverterterminals (0 ft cable) and at the motor (Vrated = 400 V)terminals with different cable lengths are presented next. Theovershoots also depend on the type of cable used in theinstallation; therefore the waveforms shown below areillustrative only.

Vpeak= 560 V

Vpeak= 750 V

Partial discharge effect on the motor insulation system

Damaged insulation due to PD activity

PD is thus a low energy discharge which, after long termactivity, prematurely degrades the motor insulation. Theerosion reduces the thickness of the insulating material,resulting in a progressive reduction of its dielectric properties,until its breakdown voltage capability falls below the level ofthe applied voltage peak, then the insulation breakdownoccurs.

Vpeak= 630 V

Vpeak= 990 V


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When the time between successive pulses is less than 6s,particularly when the first and the last turns of a single coil ofa random winding are side by side, it may be assumed thatthe voltage between adjacent conductors is the peak to peakvalue between pulses. This fact results from the rapidity of

the pulse propagation within a coil, because while the firstturn stands a peak to peak voltage value, the voltage on thelast turn is very low, probably zero.In the case of the example shown above the MTBP wasbelow 6s and there were actually motor failures due toshort circuit between turns.

6.5.4 Switching frequency (fs)

Beside the effects caused by the rise time and the MTBP,there is also the frequency at which they are generated.Differently from eventual impulses caused by line handles, itis about a pulse train supported at a certain frequency.Owing to the fast developments on power electronics,presently this frequency reaches easily values such as 20kHz. The higher the switching frequency, the faster thedegradation of the motor insulation takes place. Studies bearout that there is no simple interrelation between the insulationlife and the switching frequency, in spite of that experienceshave shown interesting data: If fs 5 kHz the probability of insulation failure occurrence

is directly proportional to the switching frequency If fs > 5 kHz the probability of insulation failure occurrence

is quadratically proportional to the switching frequency.

High switching frequencies can cause bearing damages. Onthe other hand, switching frequency increase results in themotor voltage FFT improvement and so tends to improve themotor thermal performance besides reducing noise.

6.6 Criteria regarding the insulation system of WEG


When WEG low voltage induction motors are used withinverters, the following criteria must be attended in order toprotect the insulation system of the motor. If any of theconditions below are not satisfied, filters must be used.

NOTE: Applications with motors rated for use in hazardousareas must be particularly evaluated - in such case pleasecontact WEG.

6.5.5 Multiple motors

If more than one motor is connected to a control, there canbe additional overshoot due to reflections from each motor.The situation is made worse when there is a long length oflead between the control and the common connection ofmotors. This length of lead acts to decouple the motor from

the control. As a result, reflections which would normally beabsorbed by the low impedance of the control can be carriedto another motor and add to the overshoot at its terminals.

6.7 Normative considerations about the insulation

system of inverter fed motors

NEMA MG1 if the voltage at the inverter input does noexceed the motor rated voltage and if the voltage observedat the motor terminals does not exceed the limits shownbelow, it may be assumed that there will be no voltagestress reducing significantly the life of the insulation system.

When connecting multiple motors to a single inverter, L mustbe as short as possible.

The maximum recommended switching frequency is 5 kHz.

Moisture is detrimental to insulating materials and therefore

must be avoided for a longer motor life to be guaranteed. Inorder to keep the motor windings dry, it is recommended theuse of heating resistors.The insulation system to be used in each case depends onthe motor rated voltage range and on the frame size.

Motor rated voltage

VoltageSpikes

motor

terminals

dV/dt

inverter

terminals

Rise

Time do

conversor*

MTBP*

VNOM 460 V 1600 V 5200 V/s

0,1s 6s460 V < VNOM 575 V 1800 V 6500 V/s

575 V < VNOM 690 V 2200 V 7800 V/s

* Informed by the inverter manufacturer

INVERTER


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6.8 Recommendations for the cables connecting WEG

motors to inverters

As already mentioned the maximum peak voltage appearingat the terminals of the inverter fed motor depends on manyfactors, predominantly the cable length.

When supplying WEG motors with inverters, the followingpractical rules are suggested for the evaluation of the need ofusing filters between motor and inverter.

IEC 60034 for motors up to 500 V the insulation systemmust stand voltage peak levels as shown below. Formotors above 500 V, reinforced insulation systems must beapplied or filters shall be installed at the inverter output,aiming to increase the rise time and to limit voltage peaks.

WEG motors fully attend NEMA MG1 Parts 30 and 31.

Nema MG1 - Part 30

General purpose motors

Nema MG1 - Part 31

Definite purpose inverter fed motors

Vrated

600 V : Vpeak

1kV

Rise time 2s

Vrated

> 600 V : Vpeak

3,1 Vrated

Rise time 0,1s

Vrated

600 V : Vpeak

2,04 Vnom

Rise time 1s

Vrated

600 V : Vpeak

2,04 Vrated

Rise time 1s

IEC 60034-17

General purpose motors

IEC 60034-25

Definite purpose motors

Valid for standard motors.

It is remarkable the similarities existing between IEC andGAMBICA criteria, as well as their disparity with respect toNEMA criteria. This results from the particular definitions ofrise time and dV/dt according to each institution. One cannotice that the insulation criteria from both IEC and GAMBICAtake into account the cable length, information which WEGalso considers relevant.

The output reactor is necessary for the eddy current thatflows from inverter to earth to be limited. The input (line)reactor prevents the inverter ground fault from tripping.

The output reactor design must take account of additionallosses occurring due to current ripple and current leakage toearth, which increases as cable length rises. For long cablesand reactors designed for small currents there will be greatinfluence of the leakage currents on the reactor losses (andheating). The cooling system of the inverter panel must alsotake the reactors additional losses into account for a safetemperature operation to be assured.

The output reactor must be installed near the inverter, asshown below.

A: Valid for motors up to 500 Vac (without filters)B: Valid for motors up to 690 Vac (without filters)C: Measured results at 415 Vac supply with different cablelengths

GAMBICA/REMA the European association of motors(REMA) and inverters (GAMBICA) manufacturers set the

criteria shown next based on its members experience.

Cable length L Output filters

L 100 m Not needed

100 m < L 300 mOutput reactor needed

(at least 2% voltage drop)

L > 300 m Special filters needed (contact WEG)


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L1 = Line reactor selection criteria according to clause 5.2L2 = Output reactor must be installed next to the inverter.

Cable shield must be grounded at both ends, motor andinverter. Good EMC practices such as 360 bonding of theshields are recommended, in order for low impedance for

high frequency to be provided.

For the shield to operate also as protective conductor, itshould have at least 50% of the phase conductorsconductance. If the shield does not have enough cross-section for that, then a separate earth conductor is neededand the shield provides EMC and physical protection only.The shield high-frequency conductance should be at least10% of that of the phase conductors.

PE = protective earth conductorSCU = concentric copper (or aluminum) screen

Symmetrical Shielded Cables: three-core cable (with orwithout conductors for protective earth) symmetricallyconstructed + a concentric copper or aluminum protectiveshield/armour

Afe = steel or galvanized iron

6.8.1 Cable types and installation recommendations

The characteristics of the cable connecting motor andfrequency inverter, as well as its interconnection and physicallocation, are extremely important to avoid electromagneticinterference in other devices.

6.9 Influence of the inverter on the motor shaft voltage

and bearing currents

The advent of static inverters aggravated the phenomenon of

induced shaft voltage/current, due to the unbalancedwaveform and the high frequency components of the voltagesupplied to the motor. The causes of shaft induced voltageowing to the PWM supply is thus added to those intrinsic to

6.8.1.2 Shielded cables

They help to reduce the radiated emission through themotor cables in the Radio Frequency range (RF).

They are necessary when the installation must comply with

the EMC Directive 89/336/EEC as per EN 61800-3. They are also necessary when using Radio Frequency

Interference Filter (whether built-in or external) at inverter

input. Minimum distances between motor cables and other

electrical cables (for instance, signal cables, sensor cables,etc.) must be observed in the final installation, as per tablebelow.

6.8.1.3 Installation recommendations

IEC 60034-25 presents cable types and construction details.

6.8.1.1 Unshielded cables

Three-core unshielded motor cables can be used whenthere is no need to fulfill the requirements of the EuropeanEMC Directives (89/336/EEC).

Certain minimum distances between motor cables andother electrical cables must be observed in the finalinstallation. These are defined in the table below.

Emission from cables can be reduced if they are installedtogether on a metallic cable bridge which is bonded to theearthing system at least at both ends of the cable run. The

magnetic fields from these cables may induce currents innearby metalwork leading to heating and increasing losses.

Recommended separation distances between motor cable

(shielded or not) and other cables of the installation

Cable Length Minimum separation distance

30 m 10 cm

> 30 m 25 cm

The basic given recommendations are summarized in thetable below. For more details and updated information thecurrent standard version shall be consulted.

The grounding system must be capable to provide good

connections among equipments, for example, betweenmotor and inverter frame. Voltage or impedance differencesbetween earthing points can cause the flow of leakagecurrents (common mode currents) and electromagneticinterference.

Examples of shielded cables recommended by IEC

60034-25

Alternate motor cables for conductors up to 10 mm2

L1

L2L3

Scu

L1 PE

L2L3

Scu

L1

L2L3

AFe

L1

L2L3

Scu

PEPE

PE

PEs


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The sum of the instantaneous voltage values at the (threephase) inverter output does not equal to zero

This high frequency common mode voltage may result inundesirable common mode currents. Existing straycapacitances between motor and earth thus may allowcurrent flowing to the earth, passing through rotor, shaft andbearings and reaching the end shield (earthed).

Practical experience shows that higher switching frequenciestend to increase common mode voltages and currents.

These discontinuous electric discharges wear the racewaysand erode the rolling elements of the bearings, causing smallsuperimposing punctures. Long term flowing dischargecurrents result in furrows (fluting), which reduce bearings lifeand may cause the machine to fail precociously.

6.9.1 Common mode voltage

The three phase voltage supplied by the PWM inverter,differently from a purely sinusoidal voltage, is not balanced.That is, owing to the inverter stage topology, the vector sumof the instantaneous voltages of the three phases at theinverter output does not cancel out, but results in a highfrequency electric potential relative to a common referencevalue (usually the earth or the negative bus of the DC link),hence the denomination common mode.

6.9.2 Equivalent circuit of the motor for the high

frequency capacitive currents

The high frequency model of the motor equivalent circuit, inwhich the bearings are represented by capacitances, showsthe paths through which the common mode currents flow.

The rotor is supported by the bearings under a layer of non-conductive grease. At high speed operation there is nocontact between the rotor and the (earthed) outer bearingraceway, due to the plain distribution of the grease. Theelectric potential of the rotor may then rise with respect to theearth until the dielectric strength of the grease film isdisrupted, occurring voltage sparking and flow of dischargecurrent through the bearings. This current that circulateswhenever the grease film is momentarily broken down isoften referred to as the capacitive discharge component.There is still another current component, which is induced bya ring flux in the stator yoke and circulates permanently

through the characteristic conducting loop comprising theshaft, the end shields and the housing/frame, that is oftencalled the conduction component.

Cer : Capacitor formed by the stator winding and the rotorlamination (Dielectric = airgap + slot insulation + wireinsulation)

Crc : Capacitor formed by the rotor and the stator cores(Dielectric = airgap)

Cec : Capacitor formed by the stator winding and the frame(Dielectric = slot insulation + wire insulation)

Cmd e Cmt : Capacitances of the DE (drive end) and the NDE(non-drive end) bearings, formed by the inner andthe outer bearing raceways, with the metallicrolling elements in the inside. (Dielectric = gapsbetween the raceways and the rolling elements +bearing grease)

ICM : Total common mode current

Ier : Capacitive discharge current flowing from the stator tothe rotor

Ic : Capacitive discharge current flowing through the bearings

the motor (for instance, electromagnetic unbalance causedby asymmetries), which as well provoke current circulationthrough the bearings. The basic reason for bearing currentsto occur within an inverter fed motor is the so called commonmode voltage. The motor capacitive impedances becomelow in face of the high frequencies produced within theinverter stage of the inverter, causing current circulationthrough the path formed by rotor, shaft and bearings back toearth.


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Crater occasioned by electroerosion on theinner raceway of the bearing

Bearing raceway damaged by bearingcurrents flow

Fluting caused by electric discharges withinthe bearing

Without bearing protection:

With protected bearing:

Without bearing protection:

6.9.3 Methods to reduce (or mitigate) the bearings

currents in inverter fed motors

For the motor bearing currents to be impeded to circulate,both the conduction (induced on the shaft) and the capacitivedischarge (resultant from common mode voltage)components must be taken into account. In order to eliminatethe current flowing through the characteristic conductingloop it is enough to isolate the motor bearings (only one of

them, in the case of a single drive end, or the both of them, inthe case of two drive ends). However, for the capacitivecomponents to be withdrawn it would be also necessary toisolate the bearings of the driven machine, in order to avoidthe migration of electric charges from the motor to the rotorof the driven machine through their shafts, which areelectrically connected in the case of direct coupling. Anotherway of extinguishing the capacitive discharge currentcomponent consists of short circuiting the rotor and themotor frame by means of a sliding graphite brush. This way,the inductive current component flowing through thecharacteristic conducting loop can be eliminated byinsulating just a single bearing of the motor, while the

capacitive current component, as well as the transfer ofcapacitive charges to the driven machine, can be eliminatedby use of a short circuiting brush.

Motor with two drive ends

Motor with one drive end

With bearing protection:


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6.10 Criteria regarding protection against bearing

currents (shaft voltage) of WEG motors on VSD

applications

6.11 Normative considerations about the current

flowing through the bearings of inverter fed motors

6.12 Influence of the inverter on the motor acoustic

noise

NEMA MG1 Part 31 with sinusoidal supply shaft voltagesmay be present usually in motors of frame 500 and larger.(...) More recently, for some inverter types and applicationmethods, potentially destructive bearing currents haveoccasionally occurred in much smaller motors. (...) Thecurrent path could be through either or both bearings toground. Interruption of this current therefore requiresinsulating both bearings. Alternately, shaft groundingbrushes may be used o divert the current around thebearing. It should be noted that insulating the motorbearings will not prevent the damage of other shaft

connected equipment. NEMA Application Guide for AC ASD Systems the

circulating currents caused by common mode voltage maycause bearing problems in frame sizes smaller than 500(most likely in the 400 and larger frames).

IEC 60034-17 for machines with frame numbers above315 it is recommended either to use an inverter with a filterdesigned to reduce the zero-sequence component of thephase voltages (so called common mode voltages) or toreduce the dV/dt of the voltage or to insulate the motorbearing(s). The need to insulate both motor bearings isseldom necessary. In such a case, the examination of the

whole drive system by an expert is highly recommendedand should include the driven machine (insulation of thecoupling) and the grounding system (possibly use of anearthing brush).

NOTE: Applications with motors rated for use in hazardousareas must be particularly evaluated - in such case pleasecontact WEG.

The rotating electrical machines have basically three noisesources: The ventilation system The rolling bearings Electromagnetic excitation

Bearings in perfect conditions produce practically despicablenoise, in comparison with other sources of the noise emittedby the motor.In motors fed by sinusoidal supply, especially those withreduced pole numbers (higher speeds), the main source ofnoise is the ventilation system. On the other hand, in motorsof higher polarities and lower operation speeds often standsout the electromagnetic noise.However, in variable speed drive systems, especially at lowoperating speeds when ventilation is reduced, the

electromagnetically excited noise can be the main source ofnoise whatever the motor polarity, owing to the harmoniccontent of the voltage.Higher switching frequencies tend to reduce the magneticallyexcited noise of the motor.

Platform Frame Size Standard Optional

W21

W22

mod < 315 IEC

mod < 504 NEMANo protect ion Please con tac t WEG

W21

W22

315 and 355 IEC

504/5 and 586/7 NEMANo protection *

Insulated bearing in

any or both motor

ends

Earthing system with

slip ring and graphite

brush between frame

and shaft

HGF

315 mod 630 (IEC)

500 mod 1040

(NEMA)

Insulated NDE bearing

Insulated DE bearing



brush between frame

and shaft

M

280 mod 1800 (IEC)

440 mod 2800

(NEMA)

Insulated NDE bearing

Insulated DE bearing



brush between frame

and shaft

IEC 60034-25 do not specify a minimum frame size onwhich bearing protection must be applied. Within theclause broaching the effects of magnetic asymmetries asshaft voltages/bearing currents cause, it is mentioned thatbearing currents commonly occur in motors above 440

kW. For other causes, no mention is made concerningframe sizes. According to the document, the solutionadopted to avoid bearing currents depends on whichcurrent component is to be avoided. It may be made eitherby means of insulated bearings or shaft grounding systemthough.

CSA 22.2 N100 Item 12 shaft earthing brushes must beused in motors of frame above IEC 280 (NEMA 440).

Gambica/REMA Technical Guide for motors of framesbelow IEC 280 the effects of bearing currents are seldomappreciable and therefore no extra protection is needed. Insuch cases, adhering strictly to the motor and drivemanufacturers recommendations regarding the

installation, cabling and grounding is enough. For framesabove IEC 280, the effects of bearing currents may besignificant and for security special protection is advisable.This may be obtained by means of insulated NDE bearingand shaft grounding system use. In such case, care mustbe taken not to bypass the bearing insulation.* For Inverter Duty l ine motors, the earthing system is standard.

6.13 Criteria regarding the noise emitted by WEG


Results of laboratory tests (4 point measurementsaccomplished in semi-anechoic acoustic chamber with theinverter out of the room) realized with several motors andinverters using different switching frequencies have shown

that the three phase induction WEG motors, when fed byfrequency inverters and operating at base speed (typically 50or 60 Hz), present and increment on the sound pressure levelof 11 dB(A) at most.


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6.14 Normative considerations about the noise of

inverter fed motors

NEMA MG1 Part 30 the sound level is dependent uponthe construction of the motor, the number of poles, thepulse pattern and pulse frequency, and the fundamental

frequency and resulting speed of the motor. The responsefrequencies of the driven equipment should also beconsidered. Sound levels produced thus will be higher thanpublished values when operated above rated speed. Atcertain frequencies mechanical resonance or magneticnoise may cause a significant increase in sound levels,while a change in frequency and/or voltage may reduce thesound level. Experience has shown that (...) an increase ofup to 5 to 15 dB(A) can occur at rated frequency in thecase when motors are used with PWM controls. For otherfrequencies the noise levels may be higher.

IEC 60034-17 due to harmonics the excitationmechanism for magnetic noise becomes more complex

than for operation on a sinusoidal supply. (...) In particular,resonance may occur at some points in the speed range.(...) According to experience the increase at constant flux islikely to be in the range 1 to 15 dB(A).

IEC 60034-25 the inverter and its function creates threevariables which directly affect emitted noise: changes inrotational speed, which influence bearings and lubrication,ventilation and any other features that are affected bytemperature changes; motor power supply frequency andharmonic content which have a large effect on themagnetic noise excited in the stator core and, to a lesserextent, on the bearing noise; and torsional oscillations dueto the interaction of waves of different frequencies of the

magnetic field in the motor airgap. (...) The increment ofnoise of motors supplied from PWM controlled inverterscompared with the same motor supplied from a sinusoidalsupply is relatively small (a few dB(A) only) when theswitching frequency is above about 3 kHz. For lowerswitching frequencies, the noise increase may betremendous (up to 15 dB(A) by experience). In somecircumstances, it may be necessary to create skip bandsin the operating speed range in order to avoid specific

resonance conditions due to the fundamental frequency.

6.15 Influence of the inverter on the mechanical

vibration of the motor

Interactions between currents and flux harmonics may resultin stray forces actuating over the motor causing mechanicalvibration and further contributing to increase the overall noiselevels. This mechanism gains importance especially whenamplified by mechanical resonances within the motor or thedriven machine. If any of the non-fundamental harmonics isnear the natural frequencies of the motor, the forcesproduced can excite vibration modes.Such effects can be attenuated with a careful design of themotor with respect to the stator and rotor slots, laminationand frame, always looking out for simplifying the mechanical

system thus reducing the possibility of exciting naturalfrequencies that develops modes of vibration within themotor.Modern frequency inverters are also provided with tools to

6.16 Criteria regarding the vibration levels presented

by WEG motors on VSD applications

Tests realized with several motors and inverters following theprocedures recommended by IEC 60034-14 confirmed thatthe vibration levels of induction motors increase when theseare fed by frequency inverters.

Furthermore, the observed increment on vibration speedsgenerally were lower with higher switching frequencies, thatis, switching frequency increases tend to reduce themechanical vibration of the inverter fed motor.

In any case, even when operating above the base speed,WEG motors presented RMS vibration velocity values (mm/s)below the maximum limits established by both the IEC60034-14 and the NEMA MG1 Part 7 standards, thusattending the criteria required.

6.17 Normative considerations about mechanical

vibration of inverter fed motors

NEMA MG1 Part 30 When an induction motor is operatedfrom a control, torque ripple at various frequencies mayexist over the operating speed range. () It is of particularimportance that the equipment not be operated longer

than momentarily at a speed where a resonant conditionexists between the torsional system and the electricalsystem (i.e., the motor electrical torque). () It also ispossible that some speeds within the operating range maycorrespond to the natural mechanical frequencies of theload or support structure and operation other thanmomentarily could be damaging to the motor and or loadand should be avoided at those speeds.

NEMA MG1 Part 31 Machine sound and vibration areinfluenced by the following parameters: electromagneticdesign; type of inverter; resonance of frame structure andenclosure; integrity, mass and configuration of the basemounting structure; reflection of sound and vibration

originating in or at the load and shaft coupling; windage. Itis recognized that it is a goal that motors applied oninverter type supply systems for variable speed serviceshould be designed and applied to optimize the reductionof sound and vibration in accordance with the preceptsexplained above. However, since many of these influencingfactors are outside of the motor itself, it is not possible toaddress all sound and vibration concerns through thedesign of the motor alone.

IEC 60034-17 The asynchronous (time-constant) torquesgenerated by harmonics have little effect on the operationof the drive. However, this does not apply to the oscillatingtorques, which produce torsional vibrations in the

mechanical system. (...) In drives with pulse-controlledinverters, the frequencies of the dominant oscillatingtorques are determined by the pulse frequency while theiramplitudes depend on the pulse width. (...) With higher

get those problems around, so that for instance specificfrequencies within the operating range can be skipped andthe acceleration/deceleration times can be convenientlyadjusted.


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pulse frequencies (in the order of 21 times the fundamentalfrequency) the oscillating torques of frequencies 6 x f1 and12 x f1 are practically negligible, provided a suitable pulsepattern is applied (e.g. modulation with a sinusoidalreference wave or space-phasor modulation). Additionally,oscillating torque of twice the pulse frequency aregenerated. These, however, do not exert detrimentaleffects on the drive system since their frequency is farabove the critical mechanical frequencies.

IEC 60034-25 If the inverter have appropriate outputcharacteristics and if due care is taken with respect to themechanical characteristics and the mounting of the motor,vibration levels similar to those resulting from sinusoidalenvironment will be produced. Therefore, there is no needfor defining vibration criteria different from those alreadyestablished in IEC 60034-14 for sinusoidal supply. Vibrationlevels measured with decoupled motors are indicative ofthe motor quality only, but in measurements accomplished

at the actual application (with the motor finally installed)rather different values of vibration levels may be obtained.

7.1 Load types

The correct dimensioning of the variable speed drive systemdepends on the knowledge of the behavior of the load, thatis, how the load is related with speed and consequently howmuch torque is demanded on the motor shaft. In mostprocesses the load may be described by one of the followingterms: variable torque, constant torque and constanthorsepower.

7.1.2 Constant torque loads

Typical examples: Screw compressors Reciprocating compressors Positive displacement pumps Extruders Crushers Ball mills Conveyors Augers Process lines (strip, web, sheet)Machines that are high impact loads (intermittent torqueloading not as function of speed, requiring that the motor andcontrol combination produce sufficient accelerating torque toreturn the load to the required speed prior to the beginning ofthe next work stroke) or duty cycle loads (discrete loads - atchanging or constant speeds - applied for defined periods oftime repeated periodically) typically fall into the constanttorque classification.

Load torque remains constant throughout the speed range Horsepower changes linearly with operation speed Rated load torque and horsepower at base speed

7.1.1 Typical examples:

Typical examples: Centrifugal pumps Centrifugal fans Centrifugal blowers Centrifugal compressors

Variable torque loads are good candidates to apply VSDs forenergy savings, once that the mechanical power available atthe motor output will not be constant - it will actually vary

suitably in accordance with the load demand, as shownbefore in Clause 3 of this technical guide.

Torque varies at a rate proportional to the square of thespeed

Horsepower varies as the cube of the speed 100% load torque and horsepower at base speed

Torque varies linearly with speed Horsepower varies as the square of the speed 100% load torque and horsepower at base speed7 Interaction between motor and

driven load

Squared torque variation

Linear torque variation


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7.1.3 Constant horsepower loads

Typical examples: Machine tools (where heavier cuts are taken at lower

speeds and lighter cuts at higher speeds) Center driven winders

Load torque drops as speed increases Horsepower results constant throughout the speed range Rated load torque and horsepower at base speed

7.2 Speed duties

7.2.1 Variable speed duty

Motors designated for variable speed duty are intended forvaried operation over the defined speed range marked on themotor and is not intended for continuous operation at a singleor limited number of speeds. The motor design takes theadvantage of the fact that it will operate at a lower

temperature at the load levels for some speeds than at otherover the duty cycle.

7.2.2 Continuous speed duty

Motors designated for continuous speed duty can beoperated continuously at any speed within the defined speedrange. The motor is designed on the principle that it may beoperated at its load level at the speed which results in thehighest temperature rise for an indefinite period of time.

8.1 Constant torque application - compressor

8.1.1 Example

Please dimension a WEG standard squirrel cage inductionmotor (TEFC) to operate with any WEG frequency inverterfrom 180 to 1800 rpm, driving a compressor demanding 34Nm of torque. Temperature rise of thermal class B (80 K)wanted.General data: Mains: 3-phase / 400 V / 60 Hz Environment: maximum temperature 40C; altitude 1000 m;

normal atmosphere Frequency inverter CFW-09: tr = 0,1 s; fchav = 5 kHz

8 Dimensioning and analysis of

actual drive system applications Practical examples

8.1.2 Solution

8.1.2.1 Regarding the temperature rise on the windings

(derating torque)

Compressors are loads that feature a constant torquedemand along the whole speed range. The motor must bedimensioned to cope with the most critical operation

condition, in this example the lowest speed within the

operating range, in which the ventilation is reduced to itsminimum while the torque demand remains constant.Considering that the operation speed may change from 180to 1800 rpm and that the base frequency is 60 Hz, then a4-pole motor must be chosen.Neglecting the slip, the demanded horsepower at the basepoint of operation is:

Nevertheless, from the thermal point of view the worstworking point of this self-ventilated motor is 180 rpm (6 Hz),which means the lowest speed and therefore the lowesteffectiveness of the cooling system of the motor within thedefined speed range. For this reason the torque deratingmust be calculated for this very condition.

According to the WEG derating criteria (subclause 6.4.1.2),when operating at 6 Hz a torque reduction of 40% results in atemperature rise of 80 oC on the motor windings.Furthermore it must be assumed constant V/f condition,because it is asked that the motor be able to operate withany WEG drive (for the optimal flux solution to be applicable,

a WEG high efficiency motor must be driven by a WEGinverter model CFW-09 version 2.40 or higher).

That is, at 180 rpm the motor will be able to supply only 60%of its rated torque. Once the load demands constant torque(equal to the torque demanded at base speed) throughoutthe operating range, the motor must be oversized inaccordance with the derating calculated.

Thus the motor rated horsepower will be:

Consulting the WEG motors catalog, the ideal motor for thisapplication is the 11 kW (15 hp) - 4 pole - 60 Hz - frame IEC132M (NEMA 215 T).

The use of forced cooling system would be an alternativeoption. In this case, motor oversizing is not needed and amotor rated 7,5 kW (10 hp) 4 pole (frame IEC 132S/NEMA213T) would satisfactorily attend the application needs.

960P(kW)

n(rpm)

34 1800 6.5 kWTL (kgfm) = P 9.81 960

==

f = 6 Hz f/fn = 6/60 = 0,10 per unitf/fn = 0,10 p.u. Tr = 0,6 per unit

56.7 10.83 kWP =9.81

=

T =T

L

Tr

34 56.7 Nm0,6

==

1800

960


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8.1.2.2 Regarding the insulation system

According to NEMA criteria the situation is the following:

Voltage at the motor terminals:

8.1.2.3 Regarding the bearings protection

According to WEG criteria regarding protection agains

Date post:	14-Apr-2018
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Induction Motors Fed by PWM (WEG)

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